Fig. 1.

The plastron of the hard-shelled turtle T. scripta. (A) Alizarin red stained embryonic plastron at G25. (B) At G22, the bridge bone from the hyoplastron extended underneath the second rib (r2), and the tip of the bridge bone grew past the rib anteriorly. The bridge extension of the hypoplastron had grown underneath the seventh rib (r7), and the tip of the bridge bone extended past the rib posteriorly. Movie S1 demonstrates the bridge extensions. (C) Hematoxylin–eosin-stained sagittal section showing an osteogenic front (arrow) of a hyoplastron bridge bone adjacent to a rib (r) at G19. e, epiplastron; en, entoplastron; fl, forelimb; hp, hypoplastron; hy, hyoplastron; x, xiphiplastron; *, bridge extensions of hyo- and hypoplastron;. (Scale bar in A, ∼0.5 cm.)